Search

The forms of arsenic (As) and lead (Pb) in coal have an important influence on their transformation during combustion. Considering that As and Pb in coal are partially associated with pyrite, pyrolysis and oxidation experiments were conducted on pyrite in a fixed-bed reactor between 600 and 1000 °C to study the transformation of As and Pb in pyrite during thermal conversion. During pyrolysis, As remained essentially unreleased below 700 °C, and the release ratio increased with increasing temperature above 700 °C. At 1000 °C, As was almost completely released into the gas phase. During oxidation, As remained essentially unreleased below 800 °C, and the release ratio increased with increasing temperature above 800 °C, reaching 61.67% at 1000 °C. The gas-phase release ratio of As showed a dynamic change that rapidly increased with time, and the change was significantly correlated with the dynamic transformation of the pyrite. During pyrolysis, As was released into the gas phase by a two-step decomposition of FeAs2. In an oxidizing atmosphere, FeAs2was oxidized to form As2O3and Fe2O3, which could further react to form FeAsO4. As was retained in the solid phase, and the release ratio of As was therefore lower. During pyrolysis, Pb was released between 600 and 1000 °C, and the release rate and release ratio increased with increasing temperature. During oxidation, Pb remained unreleased at 600 °C, and the release ratio increased with increasing temperature above 700 °C, reaching 87.69% at 1000 °C. No significant correlation was found between the release of Pb and the conversion of the pyrite, as Pb exists in the form of PbS in pyrite. During pyrolysis, Pb was released into the gas phase by the decomposition of PbS. Part of the PbS in an oxidizing atmosphere was oxidized into PbO or PbSO4, which remained in the solid phase, resulting in a relatively low release ratio.

The study of the transformation of heavy metals during the coal combustion process is of great significance to controlling their emission. In this study, three coals with different contents of arsenic (As), lead (Pb), and mineral elements were selected. The experiments were conducted at 900–1300 °C using a drop tube furnace system. The transformation of As and Pb during the combustion processes of the three coals was obtained. It was found that both the gaseous and particulate phases were important forms of As and Pb in the combustion products. As the temperature increased, the proportion of gaseous-phase As of the YBS (Yuan Baoshan) coal and JT (Jing Tai) coal first increased and then decreased, and the proportion of gaseous-phase As of the HLH (Huo Linhe) coal increased slightly. As the temperature increased, the proportion of gaseous-phase Pb of the three coals increased and reached approximately 65% at 1300 °C. For the particulate phase, As and Pb were mainly present in ultramicrometer particles after combustion. The differences in the three coal components caused the different distributions of As and Pb in the gaseous and solid combustion products. The coal with a high content of iron (Fe) and calcium (Ca) was more conducive to the migration of gaseous-phase As to solid-phase As during flue gas cooling, while the effects of Ca and sulfur (S) on Pb transformation in the coal were offset. The original modes of occurrence of As and Pb in the three coals were mainly organic-bound and residual forms. During the combustion process, the organic-bound As and Pb were completely released, the residual As and Pb were partially released, and small amounts of water-soluble and ion-exchangeable As and Pb were formed. The analysis of the distribution characteristics of As and Pb in the combustion products showed that the direct release ratios of As and Pb during high-temperature combustion were greater than 45%. During flue gas cooling, more than 60% of As and Pb in the submicrometer particle were produced by transformation from the gaseous phase to particulate phase.

Arsenic (As) pollutants are highly toxic and using Ca-based minerals to adsorb As2O3is effective for emission control. In this study, a fixed-bed experimental system was used to study the adsorption of As2O3by the mixture of CaO, SiO2, and Al2O3at 1200 °C. The amount of As adsorbed by multiple minerals was higher than that by single minerals, indicating enhanced adsorption of As2O3under the coupling effects of Ca, Si, and Al. The adsorption amount of As first increased and then decreased with an increase in the CaO:(SiO2+Al2O3) and SiO2:Al2O3ratios. Adsorbed As was mainly enriched in Ca-Si-Al mineral, including Ca2Al2SiO7. Density Functional Theory calculation was conducted to study the As2O3adsorption on the Ca2Al2SiO7(0 0 1) surface. The analysis of Mulliken charge and density of states showed that As2O3molecules formed Al-O, O-As, and Ca-O bonds with the Al, O, and Ca sites. Si and Al in Ca2Al2SiO7enhanced the adsorption ability of Ca sites by promoting electron transfer. Si/Al ratio close to 1:1 is conducive to the formation of Si-O-Al structures, thus further enhancing the adsorption. The results can help optimize coal blending strategies and modify adsorbents to further reduce As emissions during coal combustion.

Exploring the Pb retention mechanism during high-temperature coal combustion is beneficial for reducing the emission of Pb pollutants through fuel regulation. Herein, coheating experiments of ash and minerals with PbO/PbCl2were carried out. The enrichment characteristics of Pb, Na, K, and Ca were obtained through microarea composition analysis. It was determined that Pb was incorporated into the lattice of aluminosilicates by the isomorphous substitution for Na, K, and Ca. Density functional theory calculations were performed to obtain the substitution reaction paths of PbO and PbCl2on the (0 0 1) surfaces of KAlSi3O8, NaAlSi3O8, and Ca2Al2SiO7. The reaction steps included the adsorption of PbO/PbCl2, breaking of the Na–O/K–O/Ca–O/Pb–O bonds, atomic displacement, and formation of new bonds. The rate-determining steps were breaking of Na–O, K–O, Ca–O, and Pb–O bonds. The energy barrier of Pb in PbCl2substituted with K was the lowest at 34.60 kJ mol–1. Density of states analysis showed that the adsorption of PbCl2could reduce the bonding strength of Na–O and Ca–O, making it easier for Na and Ca to detach from the lattice structure and reduce the energy barrier. The order of the reaction energy barriers of the different metals substituted with Pb was K < Ca < Na. The isomorphic substitution reaction of Pb for K is most likely to occur; thus, Pb in coal combustion products is mostly enriched in K-aluminosilicate. The results can provide theoretical support for understanding the transformation of Pb in coal-fired systems and optimizing emission control technologies such as coal blending and mineral addition.